1. Technical Field
The present invention concerns a method of controlling a wind power installation. The invention further concerns a wind power installation.
2. Description of the Related Art
Wind power installations are generally known and FIG. 1 shows the general structure of a wind power installation. The aerodynamic rotor is caused to rotate by wind and thereby drives an electromechanical rotor of a generator. In that respect the present invention concerns a wind power installation which uses a synchronous generator. Thus a pole wheel or motor rotor of the synchronous generator is rotated relative to a stator of the latter. Electric current is generated in the stator by the relative rotary movement of the pole wheel with respect to the stator so that kinetic energy of the wind is converted into electric energy.
Between the pole wheel and the stator there is an air gap which represents a considerable magnetic reluctance in a magnetic circuit between pole wheel and stator. That magnetic reluctance depends in particular on the thickness of the air gap and the thickness of the air gap is therefore selected to be as small as possible. The invention concerns in particular gearless wind power installations in which therefore the pole wheel is connected to the aerodynamic rotor without an interposed transmission and rotates at the same rotary speed as the aerodynamic rotor. The usual rotary speeds here are in the range of between about 5 and 15 revolutions per minutes for relatively large wind power installations having a nominal power output of more than 1 MW. The diameter of such generators in the region of the air gap—also referred to as the air gap diameter—is usually at least several meters, that is to say at least 2 or 3 m, and can reach up to 10 m in the case of installations known at the present time. The magnitude of the air gap is also slight in such large generators and is usually only a few mm.
Any eccentricities of stator and pole wheel lead to differing thicknesses of the air gap. Elasticities of the components and thus as a result in particular the pole wheel and possibly also the stator can also lead to differing thicknesses of the air gap in the peripheral direction, more specifically in particular under the influence of mass, gravitational and magnetic forces.
By virtue of a reduction in the air gap magnitude in a given region, the magnetic reluctance decreases in the region, while the magnetic flux density increases. That in turn leads to an increased radial force density and can result in an additional reduction in the air gap thickness, this also depending on the respectively relevant elasticities. That therefore affords a boosting effect.
Contact between the pole wheel and the stator is to be avoided in any case. They are therefore to be mechanically stiffened in such a way that the magnetic forces inevitably caused by production and assembly tolerances and by material elasticities can be absorbed by the carrier structure. With an increasing diameter for the generator, the material usage caused thereby increases greatly and considerably increases the mass of the generator. That gives rise to high material costs for the generator as such, and also for the components carrying the generator, in particular the machine bearer and also the azimuth bearing which supports the machine bearer including the generator to permit wind tracking.
As general state of the art attention is to be directed to DE 10 2006 056 893 A1 and to C Patsios, A Chaniotis, E Tsampouris and A Kladas; ‘Particular Electromagnetic Field Computation for Permanent Magnet Generator Wind Turbine Analysis’, Magnetics, IEEE Transactions on, Vol 46, No 8, pages 2751-2754, August 2010.